ef-width

Arguments:
external-format

This function returns the number of octets it takes to build a
character in the argument external-format. It may
have an integer value or the value character. The
width slot of an external format is used to optimize listen techniques in a stream; by default
listen requires at least ef-width
octets to be available before returning true. Normally this means
having to go through the process of building a character, but in
simple formats, like latin1, availability of a single octet can allow
a true return from listen.